Performance, for example, charge-discharge characteristics, charge-discharge cycle characteristics, storage characteristics, and the like, of a lithium secondary battery, which has recently been under intensive research and development, is highly dependent on electrodes used for the battery. This has led to attempts to improve battery performance by improving an active electrode material.
It is known that a battery having lithium metal as a negative electrode active material can provide high energy density per weight and volume. Lithium is deposited on the negative electrode when the battery is charged, and lithium is dissolved when the battery is discharged. When charge and discharge are repeated, deposition and dissolution of lithium in the battery are also repeated. Lithium deposited on the negative electrode grows into dendrite which could cause internal short-circuiting.
A lithium secondary battery is proposed that has aluminum, silicon (Si), germanium (Ge), tin (Sn), or the like, as a negative electrode material to inhibit deposition of lithium as dendrite. These elements can electrochemically alloy with lithium when the battery is charged. These have been reported in Solid State Ionics, 113-115, p57 (1998) , and the like. Specifically, among the above described materials, Si and Ge are promising as a negative electrode active material to provide a high capacity because they have a high theoretical capacity.
The inventors of the present invention proposed a method to form an active material layer consisting of fine crystal or amorphous Si on a current collector by a film forming method, for example, CVD, sputtering, vapor evaporation, and the like. In a method of preparation of a lithium secondary battery proposed by the inventors, Si thin film was formed by sputtering and the like, to directly deposit Si on a current collector of a metallic film.
However, it is difficult to control reaction and diffusion at a boundary between the active material layer and current collector when the active material layer is formed directly on the current collector by sputtering. Especially, when an active material layer containing Si or Ge is formed on a copper (Cu) current collector, there is a problem that reaction and diffusion are excessive at a boundary between the active material layer and current collector because diffusion coefficients of Si and Ge in Cu are very large. As a result, charge-discharge characteristics of the active material layer are deteriorated. There is also a problem that a reacted product between the active material layer and current collector at the boundary makes the electrodes fragile and deteriorates charge-discharge cycle characteristics.
There is another problem that when reaction and diffusion are not performed at the boundary between the active material layer and current collector, adhesion at the boundary tends to be insufficient and the active material layer easily comes off.
Therefore, it is important to control reaction and diffusion between the active material layer and current collector at a boundary in order to manufacture electrodes having excellent charge-discharge cycle characteristics.